1. Field of the Invention
The present invention relates to coating compositions containing polyisocyanates and aspartate-terminated urea/urethane prepolymers, which may be cured to provide coatings having improved flexibility.
2. Description of the Prior Art
The reaction of polyaspartates with polyisocyanates to form polyurea coatings is disclosed in U.S. Pat. Nos. 5,126,170 and 5,236,741. The polyisocyanates are blended with polyaspartates and then reacted after the mixture has been applied to a suitable substrate to form a urea group-containing coating.
One of the deficiencies of these coatings is that they do not possess good flexibility, primarily because both of the reaction components are low molecular weight compounds. The reaction of these components results in a high concentration of urea groups, so-called "hard blocks," which are known to result in rigid coatings.
One possibility for increasing the flexibility of the coatings is to blend the polyaspartates with the known high molecular weight polyols. However, when these systems are cured the coatings obtained have a waxy feel, which makes them unsuitable for commercial applications. It is believed that this is caused by the fact that aspartate groups react with isocyanate groups much faster than hydroxy groups. Therefore, during the curing process the polyisocyanates react with the aspartates, which hardens the coatings to such an extent that the isocyanate groups cannot react with the hydroxy groups of the polyol component. The polyols remain in the cured coating as a plasticizer resulting in a waxy feel.
One method of overcoming this difficulty is to prereact the polyol with the polyisocyanate to form an NCO prepolymer. The prepolymer can then be reacted with the polyaspartate to form a fully cured coating composition. One disadvantage of this process is that unreacted monomeric polyisocyanate is also present in the NCO prepolymer. To remove this free monomer, which is necessary for environmental reasons, requires an expensive stripping process.
Copending application, overcomes the need for removing unreacted monomer by reacting the NCO prepolymers and any free monomer with polyaspartates to convert the NCO prepolymers to an aspartate-terminated urea prepolymers. These prepolymers can subsequently be reacted with a polyisocyanates to form coatings.
Even though the coatings obtained in the copending application have increased flexibility, it is an object of the present invention to further improve the flexibility of these coating, while maintaining the high crosslink density of the coatings.
This object can be achieved with the coating compositions according to the present invention that are described hereinafter. The coating compositions contain aspartate-terminated urea/urethane prepolymers, excess polyaspartates and polyisocyanates having a specific range of NCO functionalities. By using these specific coating compositions it is possible to obtain an additional increase in flexibility without sacrificing the other valuable properties of polyurethane coatings. The use of higher functionality polyisocyanates leads to an increase in the crosslink density of the coatings, which results, e.g., in excellent chemical and solvent resistance.